The present invention relates to a sensor element, particularly to a sensor such as a magnetoresistance sensor, an air flow sensor, an acceleration sensor, a pressure sensor, a yaw rate sensor, an image sensor, or the like, having a sensor face.
As a sensor element for controlling the running of vehicles, there have been used an acceleration sensor, a yaw rate sensor, a pressure sensor, an air flow sensor, a magnetoresistance sensor or the like. Among them, the air flow sensor for detecting a flow rate of gasoline is in such a constitution that, for example, temperature variation at the sensing portion in which resistance wiring is embedded caused by contacting the flow path of gasoline-containing gas is detected by the change of resistance of the resistance wiring whereby the flow rate of the gasoline-containing gas can be detected. The sensing portion is formed on a lower supporting film comprising an inorganic material such as a silicon nitride film supporting the sensing portion. The inorganic material is usually formed in a sputtering process, a CVD process, or a vapor deposition process and, therefore, film quality such as microscopic surface roughness and a film composition delicately changes depending upon apparatus and conditions for the formation of film. Due to the changes in the film quality as such, there are noted dispersion of several % in sensor characteristic (such as sensitivity) whereby it has been difficult to achieve a stable sensor characteristic with good reproducibility. There are other problems that, due to the stress difference between the sensing portion and the inorganic material, the sensor characteristic is deteriorated or a positional shift is caused in the resistance wiring constituting the sensing portion or a wiring on a contiguous control circuit. Particularly, in the case of a sensor in which wiring, such as resistance wiring, is used for the sensing portion, depending on the materials used for the wiring, adherence with a matrix material is significantly weak. When the sensor element is sealed by a resin, there poses a problem that the wiring is liable to experience a positional shift due to thermal or mechanical strain.
On the other hand, a magnetoresistance sensor is provided with a sensing portion comprising slender wires of magnetic metal, constituted to be capable of detecting a change in magnitude or direction of a magnetic field by utilizing a magnetoresistance effect of a magnetic metal. Such a sensing portion is formed on an lower layer insulating film comprising an inorganic material formed on a substrate as described, for example, in the Japanese Patent Laid-Open No.Hei.5-183145, or is formed directly on a glass substrate as described, for example, in the Japanese Patent Laid-Open No.Hei.10-270775. Therefore, like in the case of the above-mentioned air flow sensor, there are problems of dispersion of sensor characteristic and positional shift of the wiring or the like.
Further, in the conventional magnetoresistance sensor, an inorganic material such as silicon nitride film is formed as an interlayer insulating film on the outermost surface of the control circuit and, accordingly, there is a problem that, when it is attempted to make the size of the sensor element smaller by layering of the sensing portion and the surge resistance protecting from surge current, unevenness of the control circuit remains on the surface of the interlayer insulating film whereby layering the sensing portion on its surface is not possible and it is necessary to form the control circuit and the sensing portion on the same plane causing the area of the sensor element large. Further, even if the sensing portion and the control circuit are layered, it is still necessary that the sensing portion is formed on a flat surface but, since the surface of the interlayer insulating film is not flat, there is a problem that the region where the sensing portion can be formed is restricted and accordingly that the area of the sensor element becomes large.
The present invention has been achieved under such circumstances and is particularly to provide a sensor element in which a sensing portion is formed on a resin film the surface of which has a good flatness, having low stress applied to the sensing portion, capable of being formed at low temperature and having no concern of effecting adverse influence to the sensing portion even in a fabrication process.
Thus, according to the present invention, there is provided a sensor element having a sensor substrate and a sensing portion which is supported by the sensor substrate and there is a resin film between the sensor substrate and the sensing portion.
The resin film used in the present invention has a high heat resistance to the temperature for the manufacturing process and for the use of sensor element, has an excellent coverage performance of undercoat having a three-dimensional structure whereby it is possible to make its surface flat and, in addition, has an excellent close adhesion to the undercoat and to the sensing portion. Moreover, it has a low stress being able to be made into a thick film and also has a high resistance to environment.